What is time? Defining the most mysterious dimension

One way to define a non-intuitive concept is to imagine the world without it, then assess the changes its presence brings. Isaac Newton used this approach in defining force. In his laws of motion, the first law describes a universe devoid of forces, in which all objects move at constant speeds in straight lines. The second law then introduces force as the cause of departure from uniform motion, with acceleration serving as the measure of that change. In other words, to understand presence, we must first grasp absence. Taking a leaf from Newton’s book, we can apply the same principle to time.

Imagine a universe where time does not exist. In this scenario, nothing would occur because events require a shift from one state to another, necessitating a "before" and an "after." Such a universe would be a static snapshot, never aging. There would be no movement, no cause and effect, no growth, and no decay. Stars would never light up, atoms wouldn't oscillate, hearts wouldn't beat, and thoughts wouldn't emerge. Without time, development is impossible; there would only be a fixed spatial configuration that might illustrate a grand design but lacks productivity and evolution.

As time is introduced, sequences arise, resulting in transformations. The universe springs to life, weaving a tapestry of events. Causes initiate effects, and matter organizes into structures. Time is not merely a silent clock ticking passively in the background; it actively facilitates progress, learning, and history. In this way, time imparts meaning to the universe, guiding and determining its evolutionary pace. Without time, the universe would remain a meaningless, static configuration.

Once time is defined as a deviation from a static frame, the next crucial question is: how do we quantify this deviation? The challenge we encounter is that time is not a tangible entity. Unlike physical matter, whose properties we can approximate well, time cannot be examined under a microscope to understand its composition. Our experimental tools are crafted to investigate physical entities such as particles, charges, fields, and waves that interact through measurable forces. However, there are no instruments capable of detecting a particle of time, nor are there colliders capable of measuring its properties.

Adding to the complexity, time's unique nature means it behaves quite differently from everything else we know. Matter and energy follow conservation laws, which allow them to transform into one another, yet their total amount remains unchanged since the universe began. In contrast, time doesn't seem to be conserved. It looks like the universe continuously generates new outputs rather than reusing existing stockpiles, preferring to keep them as records of past events. This unique characteristic of time makes its flow irreversible. We can return to past locations, but we can't revisit past events. We can only access them through memories.

This peculiar temporal feature prompted us to seek alternative methods for measuring time intervals.  Since time doesn't repeat, we had to find repeating patterns to serve as ticking clocks. Sundials, some of the earliest timekeeping devices, relied on the consistency of Earth's orbit. They were replaced by pendulum clocks, which used Earth's gravity as a restoring force to keep them swinging. Quartz clocks measure the frequency of quartz vibrations, driven by an electric current. The most precise atomic clocks measure the superstable frequencies of light pulses associated with electron transitions between atomic energy levels.

None of these clocks measure time directly; rather, they depend on physical phenomena to observe its natural flow. Time is revealed only through the motion of matter or the evolution of a system, not as a standalone entity. Of all physical processes, motion serves as the most reliable method. It offers a precise sequence of steps that can be counted and matched against a standard. Whether it's a planet orbiting, a pendulum swinging, or a wave oscillating, each repetition maintains consistent regularity, enabling us to measure time by assessing displacement, oscillation, and periodicity.

Albert Einstein elevated the relationship between time and motion, characterized by speed, to a new level. By utilizing the constant speed of light, he extended Newton's ideas into the domain of relativity. Time measurement depends on spatial coordinates, which are merged into a unified continuum called spacetime. To keep the speed of light constant, space and time units adjust by expanding and contracting to balance each other. However, time is still not measured directly; it is inferred from physical processes such as particle behavior or the propagation of light through spacetime.

Remarkably, despite the abstract nature of time, our methods achieved extraordinary precision in its measurements. The artificial units we created, such as seconds, minutes, and hours, align astonishingly well with underlying physical reality. Atomic clocks can now measure time with errors of less than a second over billions of years. This level of accuracy allows GPS satellites to synchronize across continents, spacecraft to navigate across the solar system, and high-energy experiments to test the limits of fundamental physics. Such achievements would be impossible if our definition of time were flawed.

This leads to a profound conclusion. Even though time itself can't be observed and measured directly, we captured its rhythm with exceptional accuracy. By using motion as a proxy and linking its units to steady physical processes, we have built a bridge between abstract temporal flow and concrete experimental outcomes. In doing so, we have not only learned the practical methods of timekeeping but also how intricately it is integrated into motion, change, and the evolution of the universe itself.